As a conventional power supply unit for wire electrical discharge machining for supplying machining electric power between an electrode and a workpiece, it is known that a fine machined surface can be obtained in the workpiece by applying an ac high-frequency voltage between the electrode and the workpiece and generating electric discharge of short time durations at a high frequency of repetition. For example, it is disclosed in JP-A-61-260915 that a machined surface of 1 μm Rmax or less can be obtained by applying an ac high-frequency voltage of 1 MHz to 5 MHz between the electrode and the workpiece. In addition, it is disclosed in JP-A-7-9258 that a machined surface of 0.5 μm Rmax or less can be obtained by applying an ac high-frequency voltage of 7 MHz to 30 MHz between the electrode and the workpiece.
FIG. 10 is a block diagram illustrating an example of a conventional power supply unit for electrical discharge machining using an ac high-frequency power supply. In the drawing, reference numeral 1 denotes an electrode; 2, a workpiece; 3, a dc power supply; and 4, a high-frequency oscillation and amplification circuit. A fixed voltage or electric power is inputted to the high-frequency oscillation and amplification circuit 4 from the dc power supply 3 through an external command, which in turn generates an ac high-frequency voltage, and by applying the ac high-frequency voltage Vg between the electrode 1 and the work piece 2, thereby effecting electrical discharge machining of the workpiece 2 by electric discharge energy.
FIG. 11 is a diagram illustrating an example of a bath voltage waveform at the time of no-load in a case where an ac high-frequency voltage is applied between the electrode and the workpiece in the conventional power supply unit for electrical discharge machining using the ac high-frequency power supply. By continuously supplying the ac high-frequency voltage between the electrode and the workpiece at a frequency of, e.g., 1 MHz or higher, it is possible to obtain a machined surface whose surface roughness is very smooth.
The above-described conventional power supply unit for electrical discharge machining using the ac high-frequency power supply has a large advantage in that the surface roughness of the machined surface of the workpiece becomes very smooth. However, it has become known that in a case where this power supply unit for electrical discharge machining is used in wire electrical discharge machining, there are a number of problems in order to meet exacting requirements in the recent market. The problems of the power supply unit for wire electrical discharge machining using the ac high-frequency power supply are shown below.
(1) Straightness Accuracy Declines.
With the power supply unit for wire electrical discharge machining using the ac high-frequency power supply, since the voltage remains applied between the electrode and the workpiece, an attracting force based on electrostatic force acts between the wire electrode and the workpiece, so that a so-called “barrel shape” is formed in which a central portion of the workpiece is machined by a greater degree. Hence, there is a problem in that the straightness accuracy declines.
(2) Streaks Occur in the Machined Surface.
Since the wire electrical discharge machining using the ac high-frequency power supply is the machining of an area where the surface roughness is fine, even if there is the slightest vibration of the wire electrode, its effect imparted to the quality of the machined surface becomes noticeable. Accordingly, there is a problem in that streaks occur on the machined surface of the workpiece due to the vibration of the wire electrode occurring since the reaction force based on the electric discharge and the attracting force based on the electrostatic force acting between the wire electrode and the workpiece are not fixed. In addition, such streaks on the machined surface can be confirmed by visual observation as well.
As for the machined surface of the workpiece based on the power supply unit for wire electrical discharge machining using the ac high-frequency power supply, it is not rare that a large difference occurs between the surface roughness in the vertical direction (direction parallel to the wire electrode during machining) and the surface roughness in the horizontal direction (direction perpendicular to the wire electrode during machining). For example, there are cases where the surface roughness in the horizontal direction is aggravated by 30 to 40 percent or thereabouts in comparison with the surface roughness in the vertical direction. FIGS. 12A and 12B shows an example of the surface roughness of the machined surface of the workpiece in a case where a 20 mm-thick steel is machined with a 0.2 mm-diameter brass wire electrode by means of the conventional power supply unit for wire electrical discharge machining using the ac high-frequency power supply. FIG. 12A shows the curve of surface roughness in the vertical direction, while FIG. 12B shows the curve of surface roughness in the horizontal direction. In the case of FIGS. 12A and 12B, the surface roughness in the horizontal direction is 1.82 μm Rmax, and the surface roughness in the vertical direction is 1.29 μm Rmax, so that it can be seen that the surface roughness in the horizontal direction is about 40% coarser than the surface roughness in the vertical direction. The difference in surface roughness based on the direction is related to the streaks occurring on the machined surface of the workpiece.
(3) The Machined Surface Roughness Declines.
With the power supply unit for wire electrical discharge machining using the ac high-frequency power supply, although the polarity of the voltage alternates, the voltage constantly remains applied, so that a phenomenon is noted in which the discharge continues for a long cycle (corresponding to several cycles to several dozen cycles). Hence, there is a problem in that only the surface roughness which is several times coarser than the surface roughness supposed to be obtained by the discharge of one pulse (half wave of an alternating current) of an ac high frequency can be obtained. FIG. 13 shows an example of a bath voltage waveform at the time of the execution of machining by the conventional power supply unit for electrical discharge machining using the ac high-frequency power supply with a power supply frequency of 13.55 MHz. The presence or absence of the discharge can be primarily determined since the peak value of the ac high-frequency applied voltage drops to a predetermined voltage or below. From the voltage waveform shown in FIG. 13, the phenomenon is observed in which the discharge takes place continuously over several dozen cycles of the ac high-frequency applied voltage. In rough machining, it is known that when the phenomenon in which the discharge occurs immediately after the application of the voltage has taken place, the discharge frequently occurs in the same location. Also in electrical discharge machining based on the power supply unit for wire electrical discharge machining using the ac high-frequency power supply such as the one shown in FIG. 13, it is estimated that discharge concentrations occur, and it is conceivable that the machined surface roughness declines due to the discharge concentrations.
As applications of wire electrical discharge machining, applications for which extremely high precision and very smooth surface roughness are required are increasing in the semiconductor industry and other industries. For example, in the machining of such as dies for IC leadframes, there are such exacting requirements as the shape accuracy being 1 μm or less and the surface roughness being 0.5 μm Rmax. To meet such exacting requirements, it has been a pressing task to overcome the above-described problems.